1. Field of the Invention
This invention relates to a composition of nonlinear optical materials which can be applied to the construction of various optical devices and a method for preparing the composition.
2. Description of the Prior Art
Materials exhibiting substantial nonlinear optical effects and capable of rapid response are required as photonics materials for use in the optical devices of the future, and therefore extensive investigations and research have been conducted in connection with the development of such materials. As possibilities for such materials, organic compounds with a .pi.-electron conjugated system are regarded as more promising than crystalline inorganic compounds wherein lattice vibrations are involved in optical behavior. An effective and generally adopted approach to the molecular design of such organic materials consists in introducing strongly electron-donative and electron-attractive substituent groups into a molecule having a .pi.-electron conjugated system. At present, many compounds synthesized in accordance with this approach have been studied. In addition, as examples of the use of polymers for such purposes, materials have been prepared by dispersing molecules with a .pi.-electron conjugated system containing electron-donative and electron-attractive substituent groups in polyoxyethylene or polyester resins, and second harmonic generation (SHG) has been observed in such materials (see e.g., Japanese Journal of Applied Physics, 27, 9, 1724 (1988); J. Chem. Soc., Chem. Commun., 250 (1988)).
However, the aforesaid approach to molecular design entails the construction of molecules with extremely large dipole moments. Consequently, owing to strong interactions between these dipole moments, considering the case of two-molecule interactions, for example, these molecules are prone to form centrosymmetric crystals such that the dipole moments of paired molecules mutually cancel. In particular, the dipole moment is usually largest in such a compound that electron-donative and electron-attractive substituent groups are located at mutually parapositions on an aromatic ring, but this compound is correspondingly very prone to the formation of centrally symmetric crystals. This constitutes a problem, since such centrally symmetric crystals do not exhibit the secondary nonlinear optical effect (SHG). Furthermore, if the aforesaid compounds are dispersed in a polymer, then an electric field must be applied thereto in order to orient the molecules. Also, a problem of temporal stability arises over time. Moreover, the dispersed systems so produced may lack adequate transparency.